Silver halide color photography depends on the formation of dyes in order to reproduce an image. These dyes are typically formed from couplers present in or adjacent to the light sensitive silver halide emulsion layers which react to image light upon exposure. During development, the latent image recorded by the silver halide emulsion is developed to amplify the image. During this process in which silver halide is reduced to elemental silver, the color developer compound used is at the same time oxidized, as is typical in a redox reaction. The oxidized developer then reacts or couples with the coupler compound present in or adjacent to the emulsion layer to form a dye of the desired color.
Typically, a silver halide emulsion layer containing a magenta dye-forming coupler is sensitized to green light. This facilitates so-called negative-positive processing in which the image is initially captured in a negative format where black is captured as white, white as black, and the colors as their complimentary colors (e.g., green as magenta, blue as yellow, and red as cyan). Then the initial image can be optically printed in the correct colors on a reflective background through the device of optical printing which has the effect of producing a negative of the negative, or a positive image of the original scene.
Viewable images may also be produced through reversal processing in which the initial negative image is reversed by using a black and white developer, processing to remove the developed silver but leave the undeveloped silver halide, and then fogging the element in the presence of color developer to provide developed silver in inverse proportion to the amount of image light, with corresponding dye formation.
For incorporation into a photographic element, the couplers are typically dissolved in high-boiling organic solvents known as "coupler solvents," and dispersed in gelatin with the aid of surfactants.
One of the difficulties with color couplers is achieving simultaneously all of the required physical and chemical properties of the coupler and the dye formed from it. For instance, the coupler must have good solubility in the coupler solvent, good dispersibility in gelatin, and high dye-forming activity. It must also have a high degree of resistance to decomposition due to light, heat and humidity, which can cause stains. The image dye must have the proper hue and must have a high degree of resistance to fading or hue changes due to light, heat and humidity.
Couplers that form magenta dyes upon reaction with oxidized color developing agent are described in such representative patents and publications as: U.S. Pat. Nos. 2,311,082, 2,343,703, 2,369,489, 2,600,788, 2,908,573, 3,062,653, 3,152,896, 3,519,529, 3,758,309, and 4,540,654; and "Farbkupplereine Literature Ubersicht," published in Agfa Mitteilungen, Band III, pp. 126-156 (1961). Typically, such couplers are pyrazolones and pyrazoloazoles, especially pyrazolo[2,3-b][1,2,4]triazoles described by Formula (A) and pyrazolo[3,2-c][1,2,4]triazoles described by Formula (B). ##STR2##
In Formulas (A) and (B), R and R.sub.2 represent substituents and Z is a hydrogen atom or a group capable of being split off during the coupling reaction. Typically, R.sub.2 is an alkyl group. An alkoxy group in this position leads to image dyes with very poor light stability.
The present invention is concerned with the pyrazolo[1,5-a]benzimidazole type of magenta dye-forming couplers (hereafter referred to as PBI couplers). These couplers may be described by Formula (1): ##STR3## in which R.sub.2 and R.sub.5 -R.sub.8 represent hydrogen or substituents and Z represents a hydrogen atom or a group capable of being split off during the coupling reaction.
German patent 1,070,030 discloses PBI couplers that form magenta dyes upon coupling. In the examples given, R.sub.2 represents an alkyl or phenyl group. Couplers of these types are known to have poor coupling reactivity and to yield image dyes whose absorption spectra are too bathochromic for practical use in color photographic papers, and which have poor stability to light. International Patent Application WO 91/14970 describes PBI couplers with specifically substituted alkylthio coupling-off groups, including carboxyalkylthio groups. Such couplers offer marked improvements in coupling reactivity but do not offer improved dye hue or light stability. U.S. Pat. No. 5,143,821 discloses PBI couplers in which R.sub.2 represents an alkoxy group. Such couplers are advantageous because they have much better coupling reactivity than those in which R.sub.2 represents an alkyl group, and the image dyes formed from them have good spectral absorption characteristics. The dyes from these couplers have better light stability than the dyes from PBI couplers in which R.sub.2 is an alkyl group; however, the light stability of the dyes from these alkoxy PBI couplers is still not sufficient to meet the stringent requirements of future photographic products, especially color photographic papers.
Many classes of compounds are known to improve the light stability of image dyes when incorporated in the photographic element in combination with the coupler. Examples of such "light stabilizers" or "discoloration inhibitors," as they are commonly known, are 6-hydroxychromans as described, for example in U. S. Pat. No. 3,467,772; spiroindanes as described, for example in U. S. Pat. No. 4,360,589; methylenebisphenols as described, for example in British patent 1,529,908, U. S. Pat. Nos. 3,770,455, and 5,063,148; and so forth. However, none of these types of stabilizers have been found to provide sufficient improvement in the light stability of image dyes from PBI couplers to meet the stringent requirements of future photographic products, especially color photographic papers.
Light stabilizers of the type represented by Formula (S) are disclosed in U.S. Pat. No. 4,880,733 and U.S. Pat. No. 5,561,037. The substituents Z.sub.1 and Z.sub.2 are alkylene groups of 1-3 carbon atoms such that together they contain 3 to 6 carbon atoms, and R.sub.3 is an aryl or heterocyclic group, where q is 1 or 2. ##STR4## One of the embodiments of the stabilizer of Formula (S) is the thiomorpholinedioxide represented by Formula (S-1): ##STR5## wherein R.sup.1' is alkyl; R.sup.1" is alkyl, alkoxy, alkylthio, amido, ureido or halogen; x is 0 to 4; and y is 1 or 2.
U.S. Pat. No. 5,561,037 also discloses the use of co-stabilizers of Formula I: ##STR6## wherein each R.sub.4 independently represents an alkyl or alkoxy group of 1 to 32 carbon atoms and p is 1 to 4; Y is --NHSO.sub.2 -- or --SO.sub.2 NH--, and R.sub.5 is an alkyl group of 1 to 16 carbon atoms.
A problem to be solved is to provide a photographic element containing a light-sensitive silver halide emulsion layer having associated therewith a pyrazolo[1,5-a]benzimidazole type of magenta dye-forming coupler that yields a magenta dye with improved stability to light.